Various types of synthetic fibers having different surface structures, as well as the processes for obtaining such fibers, are known to the art.
U.S. Pat. Nos. 3,102,323 and 3,184,369 disclose a process for treating unstretched filaments, by passing the filaments into a cracking agent bath (such as a bath of acetone, dimethyl formamide or dimethyl sulfoxide) to produce cracked fibers, and then stretching the cracked fibers to produce nodular fibers. British patent 1,199,385 discloses immersing a still molten filament in a crystallizing agent, and then stretching the surface-crystallized filaments to produce filaments having irregular rough surfaces with protuberances and ribs. French patent 1,078,949 discloses apparatus and process for treating filaments in the solidified state. The filaments are treated with a swelling agent, a hot fluid, and then a cold fluid to obtain a filament having a porous surface.
Products produced by the prior art possess various physical characteristics and particularly a coefficient of friction (filament on filament) which facilitates the manufacturing operations utilizing threads made from such filaments. Such filaments are capable of being used as brush bristles, ropes, non-woven fabrics, and the like. However, these filaments do not possess the desired qualities such as a dull appearance, natural touch and a flexibility for making garments which have sufficiently high physical properties. There prior art products are generally obtained by the action of a solvent liquid or crystallizing agent on the solidified unstretched fiber which is formed into its final shape during a subsequent stretching operation.